You know I voted for it last time so again Doomsday Machine and I also think Balance of Terror is the stronger episode. It has Mark Lenard, introduction of the Romulans and probably my favorite quote of the series:

"In this galaxy, there's a mathematical probability of three million earth-type planets...and in all the universe, three million million galaxies like this one. And in all of that, and perhaps more, only one of each of us. Don't destroy the one named Kirk."